1. Field of the Invention
The present invention relates to a radiation generating apparatus equipped with a radiation tube in an envelope filled with an insulating fluid as well as to a radiation imaging apparatus which uses the radiation generating apparatus.
2. Description of the Related Art
A radiation generating apparatus is known which includes a radiation tube housed in an envelope, where the radiation tube in turn includes an electron source and target placed in an enclosed internal space. The radiation generating apparatus generates radiation by irradiating the target with electrons emitted from the electron source.
To generate radiation suitable for radiography, it is necessary to apply a voltage as high as 40 kV to 150 kV between the electron source and target, the electron source being a cathode in the radiation tube, and irradiate the target with an electron beam accelerated to high energies. Generally, the envelope is made of a metal material, whose potential is defined to be 0 V. Consequently, a high potential difference of at least a few tens of kV is produced between the electron source and target as well as between the radiation tube and envelope. Therefore, in order to generate radiation stably for a long period of time, the radiation generating apparatus is required to have withstanding voltage characteristics that are sufficient against such high voltages.
Japanese Patent Application Laid-Open No. S61-066399 discloses a rotary anode X-ray tube apparatus which secures a withstanding voltage by filling insulating coolant oil between a rotary anode X-ray tube and an inner wall of an envelope. By allowing the insulating coolant oil to flow freely between the rotary anode X-ray tube and envelope, the X-ray tube apparatus prevents sludge from adhering to a surface of the rotary anode X-ray tube and reduces electrical discharges between the rotary anode X-ray tube and envelope.
However, with the conventional technique, electrical discharges sometimes occur between the rotary anode X-ray tube and envelope via an inflow/outflow port used to allow the insulating coolant oil to flow as well as via an X-ray emission port of the rotary anode X-ray tube. Also, there is a problem in that if the X-ray tube is damaged by electrical discharges, X-rays cannot be generated stably for a long period of time.
As a method for dealing with this problem, it is conceivable to provide a sufficiently thick insulating coolant oil layer between the rotary anode X-ray tube and the inner wall of the envelope. However, withstanding voltage performance of insulating liquids such as insulating coolant oil is more susceptible to electrode shape, electrode surface properties, temperature, impurities, convection, and the like than other insulating materials. Therefore, the insulating coolant oil layer between the rotary anode X-ray tube whose temperature becomes as high as 200° C. or more during operation and the inner wall of the envelope needs to be set to a thickness large enough to avoid electrical discharges. Consequently, the envelope grows in size, increasing the size and weight of the entire X-ray generating apparatus. Also, increases in the thickness of the insulating coolant oil layer result in increases in attenuation quantity of the X-rays passing through the insulating coolant oil layer. To make up for the attenuation quantity, it becomes necessary to increase voltage, current, and operating time, resulting in increases in power consumption.
The above problems are not peculiar to reflection-type radiation generating apparatus, and transmission-type radiation generating apparatus are subject to similar problems. Therefore, both the reflection type and transmission type are expected to downsize the apparatus by minimizing the distance between the radiation tube and envelope, secure the withstanding voltage, making electrical discharges between the radiation tube and envelope less liable to occur, and reduce the attenuation quantity of radiation.
Thus, an object of the present invention is to provide a radiation generating apparatus which, having a configuration in which a radiation tube is placed in an envelope filled with an insulating liquid, has realized downsizing of the apparatus, improvement of the withstanding voltage between the envelope and radiation tube, and reduction in the attenuation quantity of radiation as well as to provide a radiation imaging apparatus which uses the radiation generating apparatus.